Spontaneous lymphocyte proliferation (SpP), measured in vitro as the rate of [14C]thymidine incorporation in blood lymphocytes, was investigated in non-infected postoperative patients, infected postoperative patients, and healthy volunteers, with 72, 24, and 3 h of lymphocyte culture. With 24-h cultures, infected postoperative patients showed 17-fold higher SpP than non-infected postoperative patients (2527 +/- 1552 versus 151 +/- 77 cpm, mean +/- SD, P less than 0.001) and 37-fold higher SpP than healthy volunteers (P less than 0.001). Postoperative patients without infection had twice as high SpP as healthy volunteers (P less than 0.001). Lymphocytes harvested after 24 h of cell culture showed significantly higher SpP than corresponding values at 72 and 3 h, in patients as well as in healthy volunteers (P less than 0.01). Infected postoperative patients showed a higher SpP than non-infected patients after only 3 h of cell culture (270 +/- 192 versus 48 +/- 10 cpm, P less than 0.001). An inverse correlation was observed between the level of SpP and body temperature in patients with postoperative infection (r = -0.62, P less than 0.05). The results indicate that lymphocytes are activated by uncomplicated surgery and particularly by postoperative infection, and that characteristics of SpP are reproducible in short cell-culture periods, which suggests that in vitro measurements of SpP may be of value in the detection of severe postoperative infection.